Leaf disease making quiet start on firs (5)

I believe the answer is:

folio

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'leaf' is the definition.
(I know that leaf can be written as folio)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for folio that I've seen before include "Book made of folded sheets" , "Four pages" , "eaf" , "File" , "Folded sheet - or book made from such" .)

I've seen this clue in The Sunday Times and The Times.

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